Disclaimer

I will use the color coding scheme which has become common among Pathfinder build handbooks. Also note that many colored items are also links to the Paizo SRD.

Red: Bad, useless options, or options which are extremely situational.

Orange: OK options, or useful options that only apply in rare circumstances

Green: Good options.

Blue: Fantastic options, often essential to the function of your character.

Temporary Note: Paizo has recently discontinued support of their official SRD. From this point forward SRD links on RPGBOT.net will instead link to d20PFSRD.com. While Archive of Nethys is now Paizo's officially licensed partner for the purposes of serving the Pathfinder SRD, Archive of Nethys is a horribly designed website and it simply doesn't match d20PFSRD's ability to keep pace with published content and d20PFSRD's search functionality.

If you encounter any links which still point to the old SRD, please email me so that I can correct them. I also recently added a page explaining my supported content which you may find helpful. --September 15, 2018

Archetypes

This archetype is built around the Leadership feat. If your GM does not allow Leadership in your game, ignore this archetype. It also seems to assume that you are evil and manipulative, which may also be a problem for your GM.

The Cult Master could be useful in a campaign where the party does very little fighting. They have some support abilities and they're great at Diplomacy, but they give up all of the Cult Master's meaningful combat options, leaving them to rely entirely on their allies, their cohort, and their followers.

Insidious Personality (Ex): Diplomacy is much more useful than Bluff, and giving up access to Improved Feint costs you nothing useful.

Fanatical Stare (Su): Reworking Hypnotic Stare gives up what little offensive capacity the Mesmerist had and forces it into a support role. Giving up Painful Stare means that stare feats won't work, either. The bonuses you grant aren't fantastic, and you can only apply them to one ally at a time.

Cult Tricks (Su): Neither option is especially good.

Double: Mesmeric Mirror is better, doesn't require you to have an adjacent ally, and doesn't hurt your friends.

Extol: Optimizing Diplomacy is really easy, especially with the bonus from Insidious Personality. The net benefit of this trick is a +2 bonus, which really isn't worth the Trick slot.

False Healing (Su): With hours/level duration you can throw this on your allies before you start fighting stuff and dramatically reduce your party's need for hit point recovery for the day.

Faithful Followers (Ex): Leadership is a great feat, but giving up one of your few Bold Stare slots is hard because they're so important and because there's no way to get more of them.

Fanatical Devotion (Ex): If you're evil, you're likely to suffer some penalties to leadership due to your evil actions. This offsets those penalties to some degree.

Masterful Cult Tricks (Su):

Avenge Me: Rage is a mediocre buff most of the time, so only take this if you have a really good ally to put it on.

Die for Me: This can absolutely save your life, and throwing the damage onto a level 1 NPC is a small price to pay for your own survival.

Reborn (Su): Automatic ressurection is nice, but losing your cohort at this level can be really hard. It's not like 17th-level NPCs just hang around in bars waiting for you to recruit them.

In your average campaign, the Spirit Walker is a decent choice. Undead are a commonly used type of enemies across most levels of the game

Spells: Useful options for handling undead, which are a huge problem for the Mesmerist because Hypnotic Stare does not affect undead because it's mind-affecting.

Undead Inception (Su): This makes undead even easier to handle.

Continued Animation (Su): This is amazing. It only works on humanoids until level 14, but you can turn enemies into short-term pets and turn them against this former allies in the same encounter. Coordinate with your allies to focus on the subject of your Hypnotic Stare to make sure that the subject dies quickly.

Command Undead (Su): A great way to get undead pets, especially since the Mesmerist is Charisma-based.

Master of the Dead (Su): Rule Minds is great, but fincing high-level undead to dominate is much easier. High-level undead include great options like demiliches, dracolichs, and other horrifyingly powerful creatures.

Overall, the toxitician makes dangerous trades to get something that it doesn't really need. The toxitician gives up the Mesmerist's ability to stare at creatures at range to gain the ability to debuff multiple foes. This is a scary trade to make for a lightly-armored character with d8 hit points and mediocre AC at best. Your "injections" eventually gain some useful debuffs that you can add to the effects, but you can get comparable effects stare feats on a vanilla mesmerist, and there are still saving throws to resist the effects. If you want to debuff stuff, play a real spellcaster like a sorcerer. If you want to sneak around and stab people, play a rogue.

Deft Fingers (Ex): Sleight of Hand is too situational. You'll need the bonus to deliver Injection "surreptitiously", of course, but most of the time you'll be charging into combat to stab people with an injection.

Injections (Su): You know how Mesmerists are all about staring people? Now you need to walk up and stab people with needle to do the same stuff. However, injections have one major advantage: You can have multiple in effect at the same time.

Painful Injection (Su): The way this is worded is confusing, so I'll attempt to clarify. When subject of one of your injections takes damage, it takes some more damage due to Painful Injection. However, this can only be triggered by a creature other than the mesmerist that delivered the injection. This means that you're getting half your level in damage once per round. It's now automatic so you don't need to spend a free action to activate the damage, but it's a free action so that hardly ever matters.

Ability Decrease: -2 or -4 to an ability score will rarely be a significant debuff, but you may be able to handicap a spellcaster by targeting their casting ability.

Excruciating: You need all the damage you can get. You get 8 damage at 20th level, which is almost as much as the maximum of 10 damage you get from Painful Injection at 20th level.

Fortitude Decrease: Fortitude saves are typically very high, and a lot of great save-or-suck effects work on Fortitude. Getting this in place will make those effects more powerful. Of course, the target needs to fail a Fortitude saving throw to suffer this effect.

Natural Armor Decrease: This can be a helpful way to debuff troublesome foes, but you still need to find a way to hit them once to get the effect in place.

Slow: The penalties are too small. Ability Decrease will be more impactful.

Treatment Vials (Su): Creating the vials ahead of time means that you need to somehow predict the effects you'll face on a given day. This could be useful if you have someone else to deliver the vials like a familiar in the party, but otherwise the action cost will rarely be justifiable in combat.

The only good thing about this archetype is Blinding Feint. It doesn't address the Mesmerist's terrible damage output or poor AC, and it forces the Mesmerist into melee where it absolutely does not belong.

Class Skills: You only need Acrobatics for the Outmaneuver option on Dazzling Feint, and it's terrible.

Martial Weapon Proficiency (Ex): Oh wow, one weapon. Get a rapier, I guess.

Bonus Feats: If you're going to build a character around flanking, I suppose getting the feats for free is helpful.

Dazzling Feint (Su): Blinding Strike is great, but it's literally the only good option.

Binding Strike: Blinding a creature, even if it's only for one round, can make or break and encounter for your party.

Combat Maneuver: Use Blinding Strike instead. Blinding the target makes them unable to make attacks of opportunity (usually) and removes the target's Dexterity bonus to CMD.

Critical Strike: You don't deal enough damage for this to matter.

Outmaneuver: Blind the target with Blinding Strike and walk away.

Piercing Strike: This will be an average of 8 damage at most. I wouldn't take this until very high level, and even then it's hardly worth the effort.

Sloppy Defense: The bonus is tiny and Circumstance bonuses are common.

Surprise Strike: With Improved Feint you're feinting as a move action, which means that you only get your Standard action to attack with. This gets you what is effectively an iterative attack. Of course, your damage with weapons is still terrible and you need to hit with the first attack to get the second attack. Hitting with that first attack is the only thing that matters because it triggers Painful Stare.

Shimmering Body (Sp): Your AC is going to be poor, so any defensive option is welcome.